1. Field of the Invention
The invention relates generally to electronic ballast systems and, more particularly, the invention relates to an electronic ballast system that controls the illumination of a lamp by using a reference voltage which is temperature stable and unaffected by resistance process dispersion and which enables soft dimming and ON/OFF control of the lamp via a multi-function input terminal.
2. Description of Related Technology
Generally speaking, electronic ballast systems initiate a glow discharge within a gas-filled lamp, such as a conventional fluorescent lamp, and thereafter maintain a stable supply of power to the lamp to sustain the discharge. As is well known, conventional electronic ballast systems typically include an inverter circuit that supplies alternating current (AC) power to the lamp and a lamp driver circuit, which uses a pulse-width modulated (PWM) control signal to vary the amount of power that the inverter supplies to the lamp.
FIG. 1 is an exemplary schematic diagram of a conventional lamp system 5 that uses an electronic ballast (not shown) to control the illumination of a lamp LAMP. The lamp system 5 includes a power supply unit 10, a switching circuit 20 and a lamp unit 30, all of which are connected as shown. The power supply unit 10 supplies direct current (DC) power to the switching unit 20, which includes first and second power switches S1 and S2 that are alternately turned ON and OFF by the ballast to drive the lamp unit 30 with AC power, thereby illuminating the lamp LAMP.
As is well known in the art, when the first switch S1 is in an ON condition (i.e., is conducting current) and the second switch S2 is in an OFF condition, current flows from the power supply unit 10 through the first switch S1, an inductor L, the lamp LAMP, a first capacitor CL1 and a second capacitor CL2. On the other hand, when the first switch S1 is in an OFF condition and the second switch S2 is in an ON condition, current flows from the power supply unit 10 through a third capacitor CL3, the lamp LAMP, the first capacitor CL1, the inductor L and the second switch S2. As is also well known, a resonance circuit is formed by the inductor L, the first capacitor CL1 and the second capacitor CL2 when the first switch S1 is ON and the second switch S2 is OFF. Likewise, the inductor L, the first capacitor CL1 and the third capacitor CL3 form a resonance circuit when the second switch S2 is ON and the first switch S1 is OFF.
In operation, the illumination of the lamp LAMP is controlled by varying the switching frequency of the switching unit 20. In particular, the drive current supplied to the lamp LAMP may be increased (to increase the intensity of the amp illumination) by reducing the switching frequency of the switching unit 20 or, alternatively, may be decreased (to decrease the intensity of the lamp illumination) by increasing the switching frequency of the switching unit 20.
The ballast (not shown) compares a feedback voltage developed across a current sense resistor Rsense to a reference voltage to control the switching frequency of the switching unit 20. During normal operation, the ballast increases the switching frequency of the switching unit 20, which decreases the drive current supplied to the lamp LAMP, if the feedback voltage is larger than the reference voltage and decreases the switching frequency of the switching unit 20, which increases the drive current supplied to the lamp LAMP, if the feedback voltage is less than the reference voltage. Additionally, the reference voltage may be varied to provide a soft-start interval during initial power-up of the lamp system 5 and/or may be used to control a dimming operation of the lamp system 5.
As is generally known, the stable operation of the lamp system 5 depends on the stability of the above-noted ballast reference voltage. Unfortunately, conventional electronic ballast circuits are typically based on integrated circuits, which are typically influenced by resistance process dispersion and temperature variations that cause the reference voltage to be far from stable. Further, when conventional electronic ballasts control the dimming of a lamp, the reference voltage typically changes abruptly, which abruptly alters the current flowing through the lamp (and the illumination intensity of the lamp) and strains the entire lamp system. Still further, because conventional electronic ballast systems are based on integrated circuits, remote control over the operation of the ballast becomes difficult, particularly because separate terminals are typically required for performing ON/OFF and dimming control functions.